Due to strict exhaust emission regulations, there are demands to reduce nitrogen oxide (NOx) emissions and PM (Particular Matter) emissions from diesel engines. One method of reducing the NOx and PM emissions adjusts an MAF (Mass Air Flow) of fresh air supplied to the engine, and an MAP (Manifold Absolute Pressure) indicating a pressure of air supplied to the engine, to respective target values. The MAF and the MAP may respectively be detected by an MAF sensor and an MAP sensor provided on the diesel engine. The MAF and the MAP may be controlled to the respective target values by controlling an EGR (Exhaust Gas Recirculator) that adjusts the amount of exhaust recirculated to the engine and a VNT (Variable Nozzle Turbo) that adjusts the amount of exhaust output to the outside of the engine. Accordingly, the Nox and PM emissions can be adjusted by controlling the EGR and the VNT, in order to adjust the oxygen concentration, the temperature, and the pressure of the air supplied to the engine.
However, a computing power or capability of an ECU (Engine Control Unit) that controls the diesel engine is relatively low in general, due to cost restrictions or the like, and the ECU is unsuited for control that requires a long computing time.
In one example of the conventional engine control method, effects of the EGR on the MAF are assumed to be dominant, and effects of the VNT on the MAP are assumed to be dominant. For this reason, a 1-input 1-output PID (Proportional Integral Derivative) control is performed to independently control the MAF by the EGR and the MAP by the VNT. According to this example of the conventional engine control method, a response deterioration occurs due to interference between the MAF and the MAP, however, the control using the ECU is possible because of the off-line design.
However, in actual practice, the effects of the EGR on the MAF are not dominant, and effects of the VNT on the MAP are not dominant. In other words, the MAF is affected by each of the EGR and the VNT, and the MAP is affected by each of the VNT and the EGR. Hence, a first method which suppresses the interference between the MAF and the MAP, by forming a 2-input 2-output ILQ (Inverse Linear Quadratic) controller, has been proposed in Japanese Laid-Open Patent Publication No. 2012-012968, for example. In a state in which a nozzle of the VNT is totally closed or fully opened, however, the response deterioration occurs. For this reason, in the state in which the nozzle of the VNT is totally closed or fully opened, the control is switched to a 1-input 1-output control of the EGR and MAF. According to this first method, the response of the MAF is improved compared to the conventional engine control method, and the MAP is comparable to that obtained by the conventional engine control method. In addition, this first method can perform the control using the ECU due to its off-line design.
On the other hand, a second method which suppresses the interference between the MAF and the MAP, by forming a 2-input 2-output model predicting controller that takes into consideration states in which a valve of the EGR or the nozzle of the VNT is totally closed and fully open, has been proposed in Japanese Laid-Open Patent Publication No. 2012-167577, for example. According to this second method, the response of the MAF and the response of the MAP are improved compared to the conventional engine control method. However, this second method is unsuited for the control using the ECU due to its on-line design and an extremely long computing time that is required.
Hence, according to the conventional engine control method, it is difficult to improve the response of the MAF and the response of the MAP by forming a controller that takes into consideration the limiting conditions or constraints related to the totally closed state and the fully open state of the valve of the EGR and the nozzle of the VNT by off-line design.